In air make-up and air supply house systems for modern auto plant paint booths, there are three major and persistent operational and maintenance problems:
1. Range and Mode of Modulation PA1 2. Burner System "Fouling" PA1 3. Process Controller and Modulation Speed are "Non Synchronous". PA1 A. Create within the burner conduit of an idle portion of a burner substantially the same conditions as those found in the firing portion (mock burn condition); PA1 B. Ensure that the mock condition would be omnipresent and capable of "instant" introduction; PA1 C. Be intrinsically opposed from "fire on condition" of any section or sequentially tiered portion which is programmed on to meet the demands of any required process; PA1 D. Integrate and safely interlock with an approved or approvable combustion system; and PA1 E. Be suitably interlocked electrically, mechanically, and electronically to prevent air (if that were to be used) from being forced back into the live fuel section, thus preventing a "premixed" fuel-air mixture and conversely raw fuel being emitted from the air inlet. Neither of these potentially dangerous conditions could be tolerated. PA1 a burner defining a first combustion zone and a second combustion zone, the zones being in communication such that materials from one zone can have access to the other zone; PA1 a first conduit for delivering gaseous fuel to said first combustion zone; PA1 a second conduit for delivering gaseous fuel to said second combustion zone; PA1 a first flow control valve in said first conduit, PA1 a second flow control valve in said second conduit, PA1 a source of pressurized air, PA1 a third conduit connected to allow pressurized air from said source to flow through said first combustion zone in a direction and at a rate sufficient to discourage materials from said second combustion zone from entering said first combustion zone, PA1 a fourth conduit connected to allow pressurized air from said source to flow through said second combustion zone in a direction and at a rate sufficient to discourage materials from said first combustion zone from entering said second combustion zone, PA1 a third flow control valve in said third conduit, PA1 a fourth flow control valve in said fourth conduit, PA1 and means for controlling said flow control valves, said means having a first mode in which one combustion zone can receive gaseous fuel along the corresponding conduit, while simultaneously the other combustion zone receives a flow of pressurized air from said source which is sufficient to discourage entry of materials from said one combustion zone into said other combustion zone, and having a second mode in which said other combustion zone can receive gaseous fuel along the corresponding conduit, while simultaneously said one combustion zone receives a flow of pressurized air from said source which is sufficient to discourage entry of materials from said other combustion zone into said one combustion zone. PA1 delivering gaseous fuel to said first combustion zone while simultaneously allowing pressurized air from a source of pressurized air to flow through the second combustion zone in a direction and at a rate sufficient to discourage materials from said first combustion zone from entering said second combustion zone, the flow of pressurized air from said source being maintained below a level at which the pressurized air would force fuel backwards along the conduit intended to supply such fuel to said first combustion zone.
The combination of these problems plays havoc with the process involved and wastes large amounts of energy.
Collectively, air supply systems which feed auto plant paint booths range in total volume from about 750,000 cfm. to 1,250,000 cfm.
A typical air make-up or air supply house system uses a raw gas burner to heat incoming outdoor air to a set point usually in the 68 to 78.degree. F. range. Due to the inherent burner turndown limitations, the traditional burner system cannot provide tight temperature control, especially when the outdoor air temperature is only a few degrees away from the desired discharge temperature. It is not uncommon in these systems to find that the discharge air temperature deviates from the desired set point by 4 to 6.degree. F., and that the energy waste is substantial. The cause of this is inadequate burner turndown and control.
In order to combat this operational problem, split fired burners have been tried on some systems. This arrangement employs a common modular motor mechanically driving (rotating) two flow control valves in unison via a common linkage. In this type of burner configuration, the burner is divided in two. By combining the turndown capability of the two individual sections, the staged burners can provide a somewhat broader turndown range.
However this arrangement has numerous inherent problems. Firstly, at the mechanically "staged" transition point from one section to another, there is produced in all cases either a droop or conversely a spike in the fuel input, depending on whether the system is modulating "up" or "down". The reasons for this are:
A. Flow control valves are inherently non-parabolic and are not well controlled at low velocities. In order to accomplish any semblance of a transition from one section to the other it is necessary for these devices to be deliberately "misadjusted" to taper off flow at the transition point. PA0 B. There is no datum or co-relation of burner staging to that of required heat rise. PA0 C. Modulating motors driving two mechanically linked flow control valves are difficult and cumbersome to adjust. As well, the reaction speed is very slow. For example, to span minimum input to maximum and back to minimum on a 15 second motor would require 30 seconds. Correspondingly, a 2 minute motor would require four minutes, and so on.
It is considered that, in order to eliminate these problems in connection with staged burners, the contemplated system must produce a turn-down ratio of 100 to 1 and must be capable of advancing from minimum to full fire and back to minimum in under 2 seconds. Another consideration is burner fouling. Raw gas burners consist of aeration plates attached to a ported cast iron fuel conduit. The tiny fuel ports (often in the hundreds) begin to reduce in cross-sectional area due to fouling soon after a system is commissioned. This condition reduces the fuel input and correspondingly the heat produced by this section; shortly thereafter, the burner fails to light reliably and the system begins to go down on "flame failure" when the heat requirement of the process is not being met.
The "stop gap" solution for the above problem is a trial-and-error system readjustment of the controls and linkages to increase the modulated fuel flow/pressure to this section to compensate for the fouling.
This condition repeats itself ever more frequently until the burner has to either be removed and replaced, or have the tiny ports re-drilled. Re-drilling is tedious and time-consuming. Aside from the expense involved, if the cross-sectional area of the ports changes to any degree due to the re-drilling, the burner's heat input per lineal foot changes, as does its "turn-down", and this can interfere with keeping the process on set point. Process downtime of course is a major concern to the end users of this equipment.
For some time there has been an awareness of the above fouling phenomenon, which occurs when attempting to increase system turn-down by staging burners. Over the years much research has been conducted into the circumstances surrounding this phenomenon.
It has been observed that fouling tends to occur when the outdoor temperature is such that the Delta T required is within the heat rise capabilities of either burner section operating alone, with the other section turned off. Further, the higher the fuel input of the live burner, the quicker the fouling took place.
Further experimentation revealed the cause of fouling to be the recirculation and the subsequent condensation of spent combustion products within the fuel conduit and ports of the idle burner. The source of these products was the opposite section (the live burner).